This application relates to flexible, microporous, open-celled polymeric foam materials with physical characteristics that make them suitable for a variety of uses. This application particularly relates to methods of degassing the components of the high internal phase emulsions which are subsequently cured to form such foams.
The development of microporous foams is the subject of substantial commercial interest. Such foams have found utility in various applications, such as thermal, acoustic, electrical, and mechanical (e.g., for cushioning or packaging) insulators, absorbent materials, filters, membranes, floor mats, toys, carriers for inks, dyes, lubricants, and lotions, and the like. References describing such uses and properties of foams include Oertel, G., Polyurethane Handbook, Hanser Publishers, Munich, 1985, and Gibson, L. J.; Ashby, M. F., Cellular Solids. Structure and Properties, Pergamon Press, Oxford, 1988. Other uses for foams are generally obvious to one skilled in the art.
Open-celled foams prepared from High Internal Phase Emulsions (hereinafter referred to as xe2x80x9cHIPEsxe2x80x9d) are particularly useful in a variety of applications including absorbent disposable articles (U.S. Pat. No. 5,331,015 (DesMarais et al.) issued Jul. 19, 1994, U.S. Pat. No. 5,260,345 (DesMarais et al.) issued Nov. 9, 1993, U.S. Pat. No. 5,268,224 (DesMarais et al.) issued Dec. 7, 1993, U.S. Pat. No. 5,632,737 (Stone et al.) issued May 27, 1997, U.S. Pat. No. 5,387,207 (Dyer et al.) issued Feb. 7, 1995, U.S. Pat. No. 5,786,395 (Stone et al.) Jul. 28, 1998, U.S. Pat. No. 5,795,921 (Dyer et al.) issued Aug. 18, 1998), insulation (thermal, acoustic, mechanical) (U.S. Pat. No. 5,770,634 (Dyer et al.) issued Jun. 23, 1998, U.S. Pat. No. 5,753,359 (Dyer et al.) issued May 19, 1998, and U.S. Pat. No. 5,633,291 (Dyer et al.) issued May 27, 1997), filtration (Bhumgara, Z. Filtration and Separation March, 1995, 245-251; Walsh et al. J. Aerosol Sci. 1996, 27, 5629-5630; published PCT application W/O 97/37745, published on Oct. 16, 1997, in the name of Shell Oil Co.), and various other uses. The cited patents and references above are incorporated herein by reference. The HIPE process provides facile control over the density, cell and pore size and distribution, proportion of cell struts to windows, and porosity in these foams.
The physical properties of HIPE foams are governed by: (1) the properties of the polymer from which the foam is comprised, (2) the density of the foam, (3) the structure of the foam (i.e. the thickness, shape and aspect ratio of the polymer struts, cell size, pore size, pore size distribution, etc.), and (4) the surface properties of the foam (e.g., whether the surface of the foam is hydrophilic or hydrophobic). Once these parameters have been defined and achieved for a particular application, an economically attractive process for preparing the material is desired. A key aspect of this process is the rate of polymerization and crosslinking, together referred to as curing, of the oil phase of a HIPE to form a crosslinked polymer network. Previously, this curing step required that the emulsion be held at an elevated temperature (40xc2x0 C.-82xc2x0 C.) for a relatively long period of time (typically from 2 hours to 18 hours or longer). Such long cure times necessitate relatively low throughput rates, as well as high capital and production costs.
Previous efforts to devise commercially successful schemes for producing HIPE foams have involved, for example, pouring the HIPE into a large holding vessel which is then placed in a heated area for curing (see for example U.S. Pat. No. 5,250,576 (Desmarais et al.) issued Oct. 5, 1993). U.S. Pat. No. 5,189,070 (Brownscombe et al.), issued Feb. 23, 1993; U.S. Pat. No. 5,290,820 (Brownscombe et al.) issued Mar. 1, 1994; and U.S. Pat. No. 5,252,619 (Brownscombe, et al.) issued Oct. 12, 1993 disclose curing the HIPE in multiple stages. The first stage is conducted at a temperature of less than about 65xc2x0 C. until the foam reaches a partial state of cure. Then the temperature is increased to between 70xc2x0 C. and 175xc2x0 C. to effect final curing rapidly. The whole process takes about 3 hours. Another scheme to produce HIPE foams envisaged placing the emulsion on a layer of impermeable film which would then be coiled and placed in a curing chamber (U.S. Pat. No. 5,670,101 (Nathoo, et al.) issued Sep. 23, 1997). The coiled film/emulsion sandwich could then be cured using the sequential temperature sequence disclosed in the Brownscombe, et al. patents discussed above. U.S. Pat. No. 5,849,805 (Dyer, et al.) issued on Dec. 15, 1998 discloses forming the HIPE at a temperature of 82xc2x0 C. (pour temperature in Example 2) and curing the HIPE at 82xc2x0 C. for 2 hours. However, none of these approaches offer the combination of very fast conversion (e.g., in minutes or seconds) from HIPE to polymeric foam that would provide for a relatively simple, low capital process for producing HIPE foams both economically and with the desired set of properties.
The art also discloses using pressure to control the volatility of monomers that, otherwise, would boil off at a suitable polymerization/curing temperature. For example, commonly assigned U.S. Pat. No. 5,767,168 (Dyer, et al.) issued on Jun. 16, 1998, discloses the suitability of pressurization to control the volatility of relatively volatile conjugated diene monomers. However, the cure time for the foams disclosed therein is still greater than two hours so there is still substantial opportunity for substantial improvement in curing rate that would improve the economic attractiveness of HIPE foams.
Deoxygenation of components (e.g. monomers and dispersion phases) used in processes such as suspension polymerization is well known. Such components are deoxygenated in order to reduce the efficiency of polymerization inhibitors typically used to prevent premature polymer formation because typical inhibitors rely on dissolved oxygen. A method exemplary of this type is described in Japanese patent application Serial No. 06-172406, published on Jun. 21, 1994. Described therein is a method for suspension polymerization of vinyl chloride monomer where the monomer is dispersed in a degassed (deoxygenated to less than 2 ppm O2) water phase at a temperature lower than a polymerization temperature. The dispersed monomer is then raised to a polymerization temperature and an initiator is charged into the aqueous medium. This method is said to reduce the amount of scale that is deposited on the polymerization apparatus during polymerization. While such processes may use a degassed water phase, the intent of such degassification is deoxygenation (other processes use sparging to replace dissolved oxygen with nitrogen for the same purpose). Thus, there has been no recognition by the art of the desirability of degassing the phases (as opposed to deoxygenation) that are formed into a HIPE and subsequently cured into a HIPE foam for the purpose of minimizing defects (e.g. voids) in the foam.
Accordingly, it would be desirable to develop a rapid and efficient process for preparing open-celled polymeric HIPE foam materials with the desired properties. It would be further desirable for such processes to provide for substantially defect-free HIPE foams. It would be further desirable for such processes to comprise simple unit operations.
The present invention relates to a process for obtaining open-celled foams by polymerizing a High Internal Phase Emulsion, or HIPE, which has a relatively small amount of a continuous oil phase and a relatively greater amount of a discontinuous aqueous phase. The present invention particularly relates to relatively high temperature processes for curing the oil phase wherein in one or both of the oil and water phases is/are degassed. Among other benefits such degassification allows the HIPE to be heated more enabling the foam to be prepared in a much shorter interval than has heretofore been possible with a substantial reduction in the level of internal defects. This enables practical continuous production processes of HIPE foams which have to this point been made via batch processes.
The process of the present invention generally comprises the steps of: 1) forming a water in oil emulsion (HIPE) wherein the oil phase comprises polymerizable monomers; and 2) polymerizing and crosslinking the monomers at temperatures greater than 90xc2x0 C. to form a HIPE foam. Specifically, the oil phase comprises: 1) from about 85 to 99% by weight of a monomer component capable of forming a crosslinked copolymer having a Tg of about 90xc2x0 C. or lower, wherein the monomer component comprises a blend of monofunctional monomers, crosslinkng agents, and comonomers capable of modifying foam properties, and 2) from about 1 to about 20% of an emulsifier component capable of forming a stable HIPE. The aqueous phase comprises from about 0.2 to about 40% by weight of a water soluble electrolyte and an effective amount of a polymerization initiator. The volume to weight ratio of aqueous phase to oil phase is between about 8:1 and about 140:1. After polymerization, the aqueous fraction of the HIPE foam may be removed by a variety of techniques to yield the open-celled, microporous, low density product.
The curing of HIPEs in a relatively short time period at elevated temperatures allows increased production and improved economics relative to previously described methods. Either batch or continuous processes can be used. In either case, because the vapor pressure of both phases in the emulsion increases as the temperature is increased, some containment and/or pressurized system may be required to prevent volatilization of HIPE components. Such elevated pressures may also serve to reduce separation of dissolved gasses during the high temperature curing and or emulsification steps. Volatilization to form gas or vapor bubbles may create defects (e.g. voids) that degrade the fine structure, particularly the cell size distribution, of the HIPE and resulting HIPE foam, and is generally to be avoided. This may be accomplished by vacuum degassing one or both of the oil and water phases, by applying pressure from an external source such as a pump or pressurized gas cylinder, by heating the emulsion in a closed container with relatively small headspace volume, by heating a portion of the composition under the surface of the emulsion in an open container such that the xe2x80x9chydrostaticxe2x80x9d pressure prevents volatilization of the liquid comprising the emulsion, or by any other method or device generally known to those skilled in the art. Elements of these approaches may be combined to develop a suitable process for rapid curing of HIPE foams.